The present invention relates to a withstand voltage tester, and more specifically to a test circuit for a withstand voltage tester, which has a safety means to keep the operator from the risk of suffering a sustained electric shock.
When electric appliances are manufactured, they must be examined through a series of safety compliance tests before delivery. The withstand voltage test is a compulsory production test on electric appliances. The test voltage for such a withstand voltage test can be as high as 5,000V. Because an operator may have to test a large number of electric appliances within a short length of time, an accidental contact of the hand with the test circuit may happen, thereby causing an accidental electric shock.
FIG. 1 shows a test circuit 1 for a withstand voltage tester according to the prior art. The test circuit 1 comprises a high voltage source 11. The high voltage source 11 is connected to a current detection device 15 through an electric line 13. The current detection device 15 is connected to the test sample (electric appliance to be tested) 17 through an electric line 13. The current detection device 15 is also connected to a withstand voltage tester (not shown), which is in turn connected to the high voltage source 11. The test circuit 1 is also connected to the grounding terminal 18 through the electric line 13. When the body 2 touches the test circuit 1 accidentally, electric current passing through the current detection device 15 is equal to the value of the electric current A1 passing through the test sample 17 plus the value of the electric current A2 passing through the human body 2. If A1=10 mA, A2=80 mA, and the preset trip current of the withstand voltage tester is 100 mA, the total current A=A1+A2=90 mA less than 100 mA. Therefore, the tester does not trip off, and the high voltage source 11 keeps outputting the high voltage A. Under this condition, the operator may subject to the risk of continuous electric shock by a high voltage.
FIG. 2 shows a test circuit 1 for another structure of withstand voltage tester according to the prior art. This test circuit 1 comprises a high voltage source 11. The high voltage source 11 is connected to a current detection device 15xe2x80x2 through an electric line 13. The current detection device 15xe2x80x2 is connected to the test sample 17 through an electric line 13. The current detection device 15xe2x80x2 is also connected to a withstand voltage tester (not shown), which is in turn is connected to the high voltage source 11. The test circuit 1 is connected to the grounding terminal 18 through an electric line 13. Normally, the electric current A1 which passes through the test sample 17 is detected by the current detection device 15xe2x80x2. However, when the human body 2 touches the test, circuit 1, the current A1 passing through the human body 2 is not detected by the current detection device 15xe2x80x2, i.e. the current detection device 15xe2x80x2 detects only the current A1 passing through the test sample 17. Therefore, the high voltage source 11 keeps outputting the high voltage current A, and the operator may subject to the risk of a continuous electric shock by a high voltage.
As indicated above, the aforesaid prior art test circuits cannot keep the operator out of the risk of suffering a sustained electric shock. If the operator touches the test circuit 1 accidentally during a test, the high voltage current A2 passes through the grounding terminal 18 to the operator""s body, causing the operator to be injured by the electric shock. The warning word of xe2x80x9cDanger! High Voltage!xe2x80x9d on the withstand voltage tester does not help to keep the operator from an electric shock.
The present invention has been accomplished to provide a withstand voltage tester which eliminates the aforesaid problem. According to the present invention, a second current detection device is installed in the test circuit and connected to the high voltage source and the overload-protective current detection device. The second current detection device comprises a switch connected to the high voltage source, and detector means connected to the ground to detect a high voltage. When the human body touches the test circuit and receives a high voltage through the ground, the detector means detects the presence of the high voltage, and outputs a signal to the switch upon such a detection, causing the switch to cut off the circuit between the high voltage source and the test sample.